Means for minimizing harmonics in radio receiver output circuits



SUTHERLIN July 4, 1933.

MEANS FOR MINIMIZING HARMONICS IN RADIO RECEIVER OUTPUT CIRCUITS Original Filed Oct. 10

INVENTOR Lee Sutherlm ATTRNEY Patented July '4, 1933 v UNITED STATES PATENT FFIC HOUSE ELECTRIC & MANUFACTURING COMPANY, A, CORPORATION oF-rENNsYL- VANIA MEANS FOR MINIIVIIZI NG I-IARMCNICS IN RADIO RECEIVER OUTPUT CIRCUITS Application filed October 10, 1928, Serial No. 311,530. Renewed. May 13, 1933.

My invention relates to electrical-discharge I devices and particularly to electrical-discharge devices which employ hotcathodes. One ofthe objects of my invention 1s to provide an electrode structure of such type that there is no perceptible fluctuation of an audible frequency in the output current of the tube.

Another object of my invention is to provide an electrical-discharge tube with a ther- .mionically-emissive electrode so rigidly spaced relative to the other electrodes therein that there is not perceptible vibration of the frequency within the range of aud b1l1ty.

A further object of my invention is to provide a radio tube having a hot-cathode filament so rigidly supported that there is no perceptible noise produced in sound-reproducing apparatus connected in the output c1rr23 cuit of said tube when the latter is subj ected to the mechanicalshocks and jars incident to its ordinary use.

Other objects of my invention will be apparent upon reading the following specification taken in connection w1th the drawing, in which Figure 1 is a view, partially in elevation and partially in longitudinal section, of a radio tube having a hot-cathode filament supported in accordance with the principles of myinvention.

Fig. 2 is a view similar to F1g. 1, but the parts in section being on line 11-11 of that figure.

Fig.- .3 is a transverse sectlonal view of the device shown in Figs; 1 and 2.

Fig. 4 is a diagramemployed 1n the 01 lowing explanation of certain features of my invention. 1

Fig. 5 is a diagrammatlc view of a circuit arrangement in which my invention may I plate'and grid electrodes-varies periodically and, since the impedance to current flow is largely dependent upon this distance, the output current of the tube undergoes sum-- lar periodic vibrations, although the supply u voltage of the source supplying it is abso lutely constant.

The more sensitive the receiving'set in which thetubes areto be used, thegreater the difiiculty resulting from fluctuations of output current, because the summation of all the fluctuations in the output currents of each tube in a receiving set appears in the current operating a loudspeaker or other sound-reproducer; and, if the t'oregoing mechanical vibrations are of audible frequency, a distinct sound, generally known as microphonic hum results. It is, accordingly, very necessary that mechanical vibrations of the electrodes generally,and particularlyof thehot cathode filaments, shall be reduced to the minimum possiblevalue, at least as regards those within the range of audible frequencies.

It has been possible, for sometime, to produce, radio tubes of sufficient capacity for use as detectors and amplifiers in household radio receiving sets which require the output of only a single dry battery cell to heat the oathodes to their operating temperature. Such tubes usually have cathodes from to A of an ampere at 1.1 volts to heat them to their best operating temperatures; Such cathodes are usually made by coating metallic-base filaments with oxides of the alkali earths and ithas been possible, by proper selection of the available materials, to produce cathode filaments to meet the requirements that have a length of approximately 1 inch.

Radio tubes of the foregoing general character have been on the market for a number of yearsand, in these, the cathode filament has been in the form of a straight wire, of the length mentioned, stretched between supports at its ends. has been surrounded by a helical grid electrode and a circular 'c'ylindrical plate electrode.

So far a I am aware, cathode filamentsof the low-voltage and current ratings specified I above have always been made straight, as above described,

r However, it has been found'that there is a perceptible hum, due to mechanical vibrations of filaments of the foregoing tubes, and,

Such a cathode filament for certain purposes, such as airplane radio sets, the fluctuations in the output current have been so severe as to produce noise of a very undesirable magnitude.

It is, accordingly, the objectof my'present invention to produce a radio tube having a cathode filament of only about one inch in length-adapted for use with'a single dry cell which shall have substantially no current fluctuations'of audible magnitude and "frequency in its output circuit caused by-mechanical vibration of said-filament.

The minimization of the mechanical vibrations necessary to this result is attained by supporting a cathode filament of .theforegoing dimensions in the form of a V on'short rigid support Wires and enclosed within grid and plate electrodes also rigidly supported and braced.

In certain instances, higher-voltage cathode filaments, which were several inches in length, have been supported in the formof a V, but the purpose of this was not theprevention of microphonic noises. This will be evident when it is considered that it is the running length of one side of the V which determines the frequency and magnitude of its mechanical vibration, and, accordingly, a V- shape filament each side of which is one inch long will produce output current fluctuations of the same frequency and twice the=magnit 'u'deof those produced bya single straight filament one'inch long. i

In the case of the above-described tubes having filaments several inches long, the object of employing the V-formation wasto shorten the end-to-end length of the radio tube itself, and thereby economize in cost and space'requirement in the radio set. Thus, the diameter of a radio tube employing a V-shape filament is not perceptibly greater than that of a radio tube employing a single straight filament; but, if a filament several inches long must be accommodated, the anode and grid structure and the tube-itself must be somewhat longer than the filament, if the latter is mounted in a straight-line position. In other Words, with a filament two inches long the tube would haveto be something over three inches'long. On'the other hand, if the filament were bent back upon itself in the form of a V, the length of the tube could be reduced by approximately one inch so that its extreme length would be only a little over two inches. 7

i In quantity of production, the saving of glass and of time required to exhaust the interior volume of the tube is a considerable item and this led to the employment of the "\l-shape where longfilaments had to be used. lVherethe filament length was only one inch, any saving of cost in the foregoing directions from the use of a V-shape filament would be more than counter-balanced by'theincreased mechanical difficulties met with in mounting hills of approximately of an-inch.

this has always been the previous practice.

It is, accordingly, a novelfeature, not appremated by prior art workers, that it'is advantageous to-mount filanients, -even as short as one inch, in the V-form, since such a mounting eliminates vibrations of'audible frequency, notwithstanding the increased .ex-

- pense incident to thedifiiculty of thus mounting shortfil-aments. I

lVith theforegoing' objects .and principles in mind, myzinvention may thus be understood by referenceto Fig. 1, wherein 1 is a radio tube of conventional fO1'11'lil21lllg.ifl

reentrant stem and a press, in whichiare sealed 7. short heavyainleading wires. The iniddle three of these wires support V-shape filament 3 having .a length betweeitits termi- The middle-'wirenof the three extends upward, parallel to the plane of the V-shapofilament. but oil'set therefronnmnd-supports the apex of the 'v' on a. resilientihook which is preferably coated =with.porcel.ainor silica to mini- 1111236COlldUCtlOl'l of heat away from the .fila

ment. The twoiinleading wires at each side of the t-hree'just mentioned support a helical grid electrode of conventional forn'i and the two outside leads supportaplate in the form ofa flattened cylinder. The support rods for the grid 4 and plate are bent through 180 degreesjustabovethe top of the anode and are tied together-with the middle support for the tip of;the":Vby a bracing member (3 of glass into which theyaare fused. Both the grid at and the plate 5 a re spaced as uniformly as possible at all..poiuts from the cathode filament 3. Jill of the inleading wires are made as sh-ort 'as possible, consistently with the difficulties of manufacture and of preventing craclring of the cellfroniheat conduction. To. give practical dimensions, I have found; :good results to follow if .allthe inleadingwvires are- 1.0 mm. in diameter and project 4 inch above the glass of thepc'ess.

The electrodestructure thus described is secured into the tube 1 in ways Weil known a approximately -square mils in cross-see tional 'area),-is coated with a mixture of equalwparts of barium oxideiand strontium oxide to the amount-of approximatelv 5 milligrams per square-centimeter of surface.

This coating may bemrppliedbymaking a llO creamy paste of the above-mentioned oxides, dipping the'filament into it, permitting it to dry and then heating it in. carbon dioxide to a full red heat. If necessary, the dipping operation should be repeated to produce the thickness of coating specified above. Such a filament will be found to be heated in vacuo to approximately 900 degrees Centigrade by ampere and to emit electrons sutlicient to supply a plate current upto 10 milliamperes before reaching the saturation point oroverheating to such an extent as on the average to decrease its operative life below 500 hours.

The cathode filament thus produced may be welded to the two inleading wires at each terminal by methods well known in the art. The ends of the three supporting wires are so spaced that the filament-is mounted in the form of a J, the two sidesthereof preferably at an angle of ai iproximately 60 degrees. The helical grid 4 may be wound from 5 mil nickel wire. :28 turns per inch, in the form of a flatten-ed cylinder, the plane faces. of which are approximately a length of inches and a breadth of inches. The anode 5 is likewise in the form of a flattened'cylinder of 5 mil sheet nickel, the plane faces of which are approximately inches apart, its length being an inch and its extreme breadth an inch. The V- shaped filament is positioned in the mid plane of both the grid and the anode cylinders. The grid electrode is of such a length as to exnd slightly above and below the ends of i1 ccathode, and the plate electrode issimiarly pcsitioned. 1

it will be found that I have provided a structure for hot cathode in an electricaldischarg tube in which the tendency to produce mechanical vibrations of audible frequency much less than in the case of the athodes of similar length, voltage and current. employed in the prior art.

While I do not wish to be limited to the correctn'ess of any theory,it is my belief that the absence of audible fluctuations in the output current of such tube-as is herein described re sults both from the decreased amplitude of vibration of the filament itself and also-from the fact that its frequency of vibration is not one to which the ear is particularly sensitive. it'is also believed that increased effectiveness in preventing microphonic fluctuations results from the emplo y'ment of a sup porting hook and rod at'the apex of the V which extend in a plane perpendiclar to that of the sides of the V, and that the seven short heavy support wires and tieir bracing are effective features in promoting the desired result.

It'will be realized that, although the foregoing dimensions are given as illustrativev of one particular form, of my invention which has been found effective in practice, these may be varied when it is necessary to make other i of an inch apart and having it may be desirable to support the'filament with more than a single apex as, for instance, in the form of a letter M. In such circum stances, the free length between supports of thesides ofthe filament would be correspondingly decreased resulting in a greater rigidity c and a higherperiod of mechanical vibration.

. here is a further potential advantage in herent in the employment of short V-s raped cathode filamentsin that their particular structure permits the use-of alternating current of commercial. frequencies for heatingthem without the production of harmonics capable of producing audible hum in the tube output circuits. It is well known in the artthat if commercial frequency alternating current is employed for heating radio tube cathode filaments of the forms and dimensions hitherto used a periodic variation of audible frequency appears in the output current. It can be shown that this variation is the resultant of four factors: v

1. The attraction of electrons emitted from the negative end of the filament by the end of the filament which is positive at any particle lar instant.

The effect of potential drop along the filament which rises'and falls in the course of the alternating current cycle and which as l the alternating heating current in its cycle.

In receiving tubes of the many types, one or more of these causesmay produce effects in the anode circuit so small as not to be of practicalimportance. It is probably only the three effects first mentioned which produce the hum actually heard in the case of ordinary receiving tubes. 1 have discovered that the instantaneous effects of the and third factors listed are opposite in kind to those produced by the second, so that the latter tends to increase the anode current at the time each of the former tends to decrease it. The

effect of the fourth, while a periodic quantity of the same frequency as the first and third, is probably dephased therefrom by nearly three quarters of a cycle so it cannot be exactly ncutralized by either of the first three. Except for very thin filaments it is negligible in mag nitudehowever.

I have further found that'by properly proportioningthediameter, length, shape, re-

sistivity, and temperature of operation of the filament, 1t 1s possible to make the aggregate of the first and third effects substantially equal as well as opposite to the second, from which it follows thatthe net effect of all is made zero. I

Turning to the various causes of hum enumerated above, their effects may be analyzed in connection with the diagram of Fig. l, which may be considered as a View looking down on the anode and cathode shown in Fig. 1. The mid-point of the cathode taken as the axis of coordinates, and distances to the right of the point are represented by the quantity +X while distances to the left thereof, arerepresented by -.X. The two ends of the filament are lo cated at the distance +L and respectively. Between the mid-point of the cathode and the plate, the B-battery, having constant electroanotive force 0,, is connected. Between the two ends of the cathode is impressed the clectro-motive force 6 13 sin pt. Considering the first cause of hum listed above, there will exist, at the surface ment, an electric force due to the voltage between opposite ends of the filament. This force will be uniform all along the filament due to the fact that both the distance and the potential diiference between points opposite each other on the two sides of the V decrease at the same rate as the apex of the V is approached. This force may as a first approximation be considered to side-track certain electrons emitted from the cathode which otherwise would flow to the anode. Because the voltage drop along the filament is small compared with that between the anode and cathode, the number of electrons thus side-tracked relatively small.

The best known formulzn for electron flow shows it to be approximately proportional to the square of the mean electric force between anode and cathode. ing with only a small correction term, the numbers of electrons side-tracked may thus be considered to be proportional to E sin pt 2 Of the total number of electrons emitted per second by the filament a certain number N. would reach the anode if not sidetracked as aforesaid. The number which will be left availab e for flow to the anode when the voltage E sin pt is impressed on the cathode may be expressed as Z 2 O KE sin pt] where E sin pt is IR drop between ends of the cathode, 2L is the distance separating the ends of the cathode filament, and K is a constant.

Turning to the second cause of hunnthe current flowing to the anodefrom the cathode of the cathode fiia- Since we are here deal-- may be calculated with the aid of Fig. 2 by he use ofthe well known Langmuir formula.

lVhere i is current from a hot filament of length Z to a. radio tube anode, e is the potential dilfereuce between the cathode and the anode and N proportional to the number of electrons available 'at the cathode.

In the case of a three-electrode tube the Langmuir formula takes the form a: e=e E sin pt In accordance with the, Langmuir formula, the current flowing to the plate from a small clement (Z0; Wlll be represented by In ordinary receiving tubes, the lilament voltage E is small compared to (sa -t e Hence, this expression may be expanded and written 2 32 L way Sm Pt The effect of the alternating magnetic field is. the third of the factors enumerated above as producing hum. In accordance with well-known electromagnetic laws an electron moving transverse to a magnetic field is deflected by a force which is perpendicular to its instantaneous movement and to the held and proportional to the product of its velocity and the magnetic field strength.

The flow of an alternating heating current in applicants V-shaped cathode produces an alternating magnetic field encircling each limb of the filament and electrons moving radially outward toward the anode are do flected from the shorter path they would normally pursue and caused to follow a longer curved trajectory. The additional space charge incident to this longer path results in a diminution of the amount of current which the constant plate voltage can draw. The diminution is dependent upon the absolute magnitude, but independent of the direction of the magnetic field at each instant; and as a first approximation it may therefore be set down as proportional to the square of the instantaneous value of the heating current. Since the filament is sub stantially a pure resistance the diminution of the electron current emanating from an elementary length, (Z of the cathode filament can therefore be written At, =mif dx -ME sin ptdw Upon integration this gives for the total anode current g. i L 1,, =N (6 H r) (W This shows that the anode current consists of direct current component a LE2 N (e +u W 2 kM-t E L] and an alternating current component (M+ E L] cos 2 pt which is of, double frequency. It will be noticed that this alternating component has the character of an increment of the direct current which would flow if the B battery alone were supplying power to the circuit.

is zero the alternating current component producing hum disappears.

Accordingly, I have conceived that it is possible to build a tube so proportioned that the quantit 1 in parenthesis shall be zero and that no hum will be produced in such a tube. For instance by varying L, the distance separating the ends of the V-shaped filament,

the hum due to the 2nd and 3rd factors aboveenumerated may be reduced to zero.

It will be evident that a variation of L is not the only way of eliminating hum by making the quantity in theparenthesis zero. Thus, since the ratio of E to 6,, and e, is not the same in the two terms, a variation of this ratio may be used to make the two terms equal. Thus, by varying the ratio of B battery voltage, the grid voltage and the filament voltage, the desired adjustment can be attained.

Figure 5 indicates a circuit suitable for employing my invention as above described, the reference characters 1, 3, a and 5 indicatin the same tube elements as in Fin. 1. An

input circuit is arranged to impress any desired modulated quantity upon the control electrode l and. the anode 5 is connected to the cathode 3 through an output transformer 7, a blocking network comprising serially connected inductance 8 and condenser 9 in multiple with inductance 10, and a current source 11. By properly proportioning the relative magnitudes of the three elements 8, 9 and 10 the fiow of undesired harmonics may be impeded. in the output transformer, and this result will be aided by a condenser 12 which is shunted between anode 5 and cathode 3 as indicated.

While, in compliance with the patent statutes, I have described one embodiment of my invention, it will be understood that this is merely illustrative andthat other embodiments of the principles thereof will be obvious to persons skilled in the art. I desire, therefore, that the following claim shall be limited only in accordance with its express terms and the state of the prior art.

I claim as my invention In a vacuum tube, the combination of a grid electrode, a plate electrode, and a V- shaped electron-emissive cathode filament, said cathode filamenthaving such resistance, normal heating current, and distance separating its ends, that the following formula is fulfilled e e p u g I2 where 13 is plate electrode operating voltage and 6,, is grid electrode operating voltage,

where M is the decrease or the plate current due to the magnetic effect arising when one volt is impressed between ends of the cathode filament, K is the decrease of plate current due to attraction of electron so the positive end of the cathode filament when one volt is impressed between ends thereof, and L is half the distance between the ends thereof.

In testimony whereof, I have hereunto subscribed my name this 5th day of October 1928.

LEE SUTHERLIN. 

